This invention concerns an air released, spring-applied brake cylinder of the type comprising a service or "duty" piston operated by the pressurized fluid, and a spring-applied brake piston (for the parking and/or emergency brake system), held in the "off" position by the pressure of the fluid acting in opposition to the reaction effort of the spring.
Existing spring type brakes comprise braking devices of the type in which a compressed-air service or "duty" brake is combined with a spring-applied brake, held in the "off" position by air pressure while the vehicle is in motion. A part from the risk of the spring-applied brake coming into action accidentally, if the air-pipe to the spring cylinder breaks, the main drawback of these combined brake systems, which otherwise offer very satisfactory regulation possibilities for the duty brake, lies in the fact that the reaction force of the spring operating the parking and emergency brake decreases rapidly as the spring is released. The distance the spring travels is often considerable, in order to take up the inevitable clearances that exist in the brake gear and between brake linings and brake surfaces, as well as the elasticity of the brake gear, as a result of the considerable forces which the linings exert on the brake surfaces. In many cases, it has been found, after several years' service, that the reaction force of the spring in such brakes, reduced by corrosion and creep affecting the spring steel, which remains under stress, reaches only between half and a third of the initial maximum design force, when in the braking position.